1. Field of the Invention
The present invention relates to a fixing device that fixes a toner image onto a recording medium by the action of heat and pressure, and to an image forming apparatus including the fixing device.
2. Description of the Related Art
As a fixing device for use in an electrophotographic image forming apparatus such as a copying machine and a printer, a fixing device of heat-roller fixing type has been in wide use. The fixing device of heat-roller fixing type includes a pair of rollers (a fixing roller and a pressure roller) that are brought into contact with each other under pressure. By means of a heating section composed for example of a halogen lamp, which is placed in each of or one of the pair of rollers interiorly thereof, the pair of rollers are heated to a predetermined temperature (fixing temperature). With the pair of rollers kept in a heated state, such as a recording paper sheet, which is a recording medium having formed thereon an unfixed toner image, is fed to a region where the pair of rollers make pressure-contact with each other (a fixing nip region). Upon the recording paper sheet passing through the pressure-contact region, the toner image is fixed to the recording paper sheet under application of heat and pressure.
Incidentally, a fixing device for use in a color image forming apparatus generally employs an elastic roller constructed by forming an elastic layer made for example of silicone rubber on a surface layer of the fixing roller. By designing the fixing roller as an elastic roller, it is possible for the surface of the fixing roller to become elastically deformed so as to conform to irregularities of the unfixed toner image, wherefore the fixing roller makes contact with the toner image so as to cover the surface of the toner image. This makes it possible to perform satisfactory thermal fixing on the unfixed color toner image that is larger in toner adherent amount than a monochromatic toner image. Moreover, by virtue of a deflection-releasing effect exerted by the elastic layer in the fixing nip region, it is possible to provide enhanced releasability for a color toner that is more susceptible to occurrence of offset than a monochromatic toner. Further, since the fixing nip region is convexly curved in a radially-outward direction so as to define a so-called reverse nip configuration, it is possible to attain higher paper-stripping capability. That is, a paper stripping action can be produced without using a stripping portion such as a stripping pawl (self-stripping action), wherefore image imperfection caused by the provision of the stripping portion can be eliminated.
Incidentally, in such a fixing device provided in a color image forming apparatus, it is necessary to make a nip width of the fixing nip region wide in order to correspond to increase in speed. One available method of increasing the fixing nip width is to increase the thickness of the elastic layer of the fixing roller and the diameter of the fixing roller. However, in a fixing roller having an elastic layer, the elastic layer can not sufficiently conduct heat, thus, in a case where a heating section is provided inside the fixing roller, there is a problem that a temperature of the fixing roller is not followed when a process speed is increased. On the other hand, when a diameter of the fixing roller is increased, there is a problem that it takes longer time to warm up or power consumption is increased.
As a fixing device provided in a color image forming apparatus to solve such problems, Japanese Unexamined Patent Publication JP-A 10-307496 (1998) discloses a fixing device of belt fixing type that is configured so that a fixing belt is supported around a fixing roller and a heating roller and the fixing roller and a pressure roller are brought into pressure-contact with each other with the fixing belt interposed therebetween. In the fixing device of belt fixing type, since the fixing belt with a small heat capacity is heated, it takes short time to warm up and it is not necessary to incorporate a heat source such as a halogen lamp in the fixing roller, thus making it possible to provide a thick elastic layer with low hardness formed of sponge rubber and the like and to secure a wide nip width.
Furthermore, Japanese Unexamined Patent Publication JP-A 2002-333788 discloses a fixing device of planar heat generating belt fixing type with a heating section as a planar heat generating element. In the fixing device of planar heat generating belt fixing type, when a heat capacity of the heating section is reduced, the planar heat generating element as the heating section directly generates heat at the same time, thus a thermal response speed is also enhanced compared to a system in which a heating roller is heated indirectly using a halogen lamp or the like and it is possible to attain further shortening of a time for warm up and more energy saving.
Japanese Unexamined Patent Publication JP-A 2006-72182 describes a fixing device of belt fixing type in which a fixing belt is supported around a fixing roller and a heating member, and the fixing roller and a pressure roller are brought into pressure-contact with each other through the fixing belt. The heating member includes a heat generating member, and a heat radiating member that transfers heat generated by the heat generating member to the fixing belt so as to heat the fixing belt. A heat pipe having a heat transport function is arranged close to the heat radiating member.
With the fixing device disclosed in JP-A 2006-72182, recording paper sheets of a small-width size pass successively through a fixing nip region, and when a paper non-passing region of the fixing belt is heated, heat of a region of the heat radiating member corresponding to the heated paper non-passing region of the fixing belt is transported through the heat pipe. Therefore, the difference in temperature on surface of the heat radiating member in the axial direction thereof is eliminated and local overheating is suppressed, such that stable image quality can be ensured. That is, in the fixing device disclosed in JP-A 2006-72182, with respect to the difference in temperature of the heat radiating member in the axial direction thereof, the heat pipe that is arranged to be in contact with the heat radiating member transports heat from the paper non-passing region at high temperature to a region at low temperature by a heat transport mechanism of the heat pipe. Therefore, the difference in temperature on the surface of the heat radiating member in the axial direction thereof is eliminated, and the fixing belt has a uniform temperature distribution in the width direction thereof. As a result, defective image quality, such as wrinkling or gloss irregularity, due to the difference in temperature can be suppressed.
In the fixing device disclosed in JP-A 2006-72182, the difference in temperature on the surface of the heat radiating member in the axial direction thereof can be eliminated; however, when the heat generating member does not heat the entire surface of the heat radiating member, the difference in temperature of the heat radiating member in the circumferential direction thereof (the rotation direction of the fixing belt) may not be eliminated, which results in deterioration in the heating efficiency of the heat radiating member for the fixing belt. The difference in temperature of the heat radiating member in the circumferential direction thereof causes an uneven temperature distribution in the circumferential direction thereof over the region of the fixing belt in contact with the heat radiating member. As a result, defective fixing occurs, which results in defective image quality, such as gloss irregularity.
In the fixing device, to reduce the warm-up time, when the heat radiating member is configured to have a low heat capacity and a high-power-density heat generating member is used, the temperature rising rate of the heat generating member increases. For this reason, unevenness of the temperature distribution on the surface of the Cheat radiating member in the circumferential direction thereof as well as the axial direction thereof becomes conspicuous. As a result, defective fixing may occur.
When a heat generating member formed of a high-power-density resistance heat generating element, which is represented by a ceramic heat generating element, is used, the difference between the temperature of the resistance heat generating element (heat generating member) itself and the temperature of the heat radiating member or the fixing belt tends to increase. When the difference in temperature extremely increases, self-suppression of energization may occur in the resistance heat generating element due to an increase in electrical resistance of the resistance heat generating element or the resistance heat generating element may be damaged due to thermal shock.
In the fixing device disclosed in JP-A 2006-72182, when the heat generating member is configured such that a resistance heat generating element forms a predetermined surface, the heat radiating member is arranged to be in contact with one surface in the thickness direction thereof from among a plurality of surfaces of the heat generating member. For this reason, in the heat generating member, there is no case where heat is lost from other surfaces which are not in contact with the heat radiating member. Accordingly, the resistance heat generating element is likely to be in an overheated state. When the resistance heat generating element is in the overheated state, self-suppression of energization occurs in the resistance heat generating element undergoes due to an increase in electrical resistance of the resistance heat generating element, such that continuous heat generation is not carried out in the heat generating member. Then, heat generation of the heat generating member becomes unstable, and heat generation efficiency is deteriorated. When the heat generation efficiency of the heat generating member is deteriorated, the warm-up time of the fixing device is extended.